1. Field of the Invention
This invention relates to a process and apparatus for two-stage combustion of fossil fuels, in particular, natural gas, which provides controllable combustion intensity, increased heat release, high combustion efficiency and low pollutant emissions. The process and apparatus of this invention are particularly suited to combustion chambers having a non-circular cross-section such as absorption chillers, process heaters, heat treating furnaces and other related equipment, as well as combustion chambers having a circular cross-section such as firetube boilers, indirect fired furnaces using radiant tubes and other related equipment.
2. Description of Prior Art
Conventional combustion of fossil fuels produces elevated temperatures which promote complex chemical reactions between oxygen and nitrogen, forming various oxides of nitrogen as by-products of the combustion process. These oxides, containing nitrogen in different oxidation states, generally are grouped together under the single designation of NO.sub.x. Concern over the role of NO.sub.x and other combustion by-products, such as sulfur oxides (SO.sub.x), carbon monoxide (CO), total hydrocarbons (THC) and carbon dioxide (CO.sub.2), in "acid rain" and other environmental problems is generating considerable interest in reducing the formation of these environmentally harmful by-products of combustion.
Natural gas is a low emission, high efficiency fuel which can help reduce these emissions. As a result, numerous ultra-low emission, natural gas-fired combustion systems are under development.
Known methods of combustion for reducing NO.sub.x emissions from combustion processes include flue gas recirculation and staged combustion. U.S. Pat. No. 4,004,875 teaches a low NO.sub.x burner for combustion of liquid and gaseous fuels in which the combustion area is divided into at least two stages and combustion products are recirculated, cooled and reintroduced into the primary combustion zone, resulting in a reduction of NO.sub.x emissions. The secondary combustion air is introduced into a secondary combustion zone downstream of the primary combustion zone in an amount sufficient to complete combustion therein. Fuel and primary combustion air are introduced into a primary combustion zone formed by a burner tile which provides a high temperature environment for the fuel and air mixture to promote combustion. Except for the opening into the secondary combustion zone, the tile is completely surrounded by a steel enclosure forming an annular space around the tile. Thus, as fuel and air are injected into the primary combustion zone, part of the partially combusted fuel and air is recirculated around the outside of the tile in the annular space between the tile and the steel enclosure and back into the upstream end of the primary combustion zone. It is known that, in addition to limiting the oxygen available in a combustion process for formation of NO.sub.x emissions, NO.sub.x emissions may also be controlled by maintaining the temperature in the combustion zone below the temperature required for formation of significant NO.sub.x, about 2600.degree. F. Although the '875 patent teaches recirculation of cooled partial combustion products from the downstream end of the primary combustion zone to the upstream end of the primary combustion zone, any heat removed from the primary combustion zone as a result of cooling and recirculating the partial combustion products is reintroduced into the secondary combustion zone, resulting in no net heat removal from the combustion process. Thus, while the burner taught by the '875 patent contains certain requisites for reducing NO.sub.x emissions, most notably flue gas recirculation and staged combustion, provisions for maintaining the temperatures in the primary and secondary combustion zones below the temperature required for formation of NO.sub.x in the combustion process are absent.
U.S. Pat. No. 4,629,413 teaches a low NO.sub.x burner utilizing staged combustion in which a mixture of primary combustion air and fuel are introduced into a primary combustion chamber and secondary combustion air is introduced into the combustion chamber in a manner such that the mixing of the secondary combustion air with the flame generated by the mixture of fuel and primary combustion air is delayed. To further inhibit the formation of NO.sub.x emissions, cooled flue gases are recirculated within the combustion chamber into the fuel-rich combustion zone at the base of the flame, that is, the upstream end of the primary combustion zone.
U.S. Pat. No. 5,044,932 also teaches a process and apparatus for reducing the NO.sub.x content of flue gas effluent from a furnace in which cooled flue gases are internally recirculated from the downstream end of the combustion chamber into the upstream end of the combustion chamber where it undergoes reaction with the flame generated by the fuel and air introduced into the upstream end of the combustion chamber. Flue gas recirculation for mixing with primary combustion air and fuel prior to initiation of combustion is taught by U.S. Pat. No. 5,092,761. Finally, U.S. Pat. Nos. 3,097,686, 4,445,842, 3,760,776, 4,699,071, 4,659,305, and 4,656,972 generally teach the concept of flue gas recirculation from the exhaust stack of a furnace system back to the burner by means external to the combustion chamber.
A combustion process producing low NO.sub.x emissions utilizing staged combustion is taught by U.S. Pat. No. 4,007,001 in which 0-65% of the total air required for combustion is introduced into a primary combustion zone and 5-25% of the total air required for combustion is provided to a secondary combustion zone. Both U.S. Pat. Nos. 4,021,188 and 3,837,788 teach staged combustion with less than a stoichiometric amount of air in the primary combustion chamber with additional air being added to the secondary combustion chamber for completion of combustion.
U.S. Pat. No. 4,575,332 teaches staged combustion in a swirl combustor with forced annular recycle of flue gases to the upstream end of the primary combustion zone, and U.S. Pat. No. 4,395,223 teaches staged combustion with excess air introduced into the primary combustion zone with additional fuel being introduced into the secondary combustion zone.
As previously stated, temperature in the primary and secondary combustion zones of a combustion chamber is a critical parameter by which NO.sub.x emissions from a combustion process can be controlled. By providing less than the stoichiometric requirement of combustion air to the primary combustion zone as taught by the prior art, temperatures within the primary combustion zone are substantially below the temperatures of a primary combustion zone into which a stoichiometric, or more than a stoichiometric, requirement of air is introduced. However, the heat generated in the primary combustion zone in accordance with known combustion processes is conveyed into the secondary combustion zone into which secondary combustion air required for completing combustion of the fuel is introduced. Thus, the net heat within the combustion chamber remains unchanged.